The present invention relates to a device and method for controlling the movement of a focusing lens in an apparatus for reading recorded information.
An optical information readout apparatus reads out information recorded on a disk by illuminating the disk surface with a pickup beam spot and receiving a beam reflected from or passing through the recorded surface. Since it is necessary to focus the spot beam accurately on the disk surface at all times, the readout apparatus is provided with a focus servo unit for controlling the position of a focusing lens.
In the focus servo unit, no focus error signal is produced when the focusing lens thereof is positioned at a distance D.sub.0 from the disk surface where the spot beam is accurately focused on the disk surface. When the distance between the focusing lens becomes greater or smaller than the distance D.sub.0, an error signal is generated which has a magnitude dependent on the amount of deviation of the focusing lens and polarity dependent on the direction of deviation of the focusing lens. The focus error signal therefore has a so-called S-curve characteristic. A motor for driving the focusing lens is controlled by the focus error signal to move the focusing lens towards or away from the disk surface for maintaining the focused condition of the spot beam correctly on the disk surface at all times, even if the disk surface is warped.
Before the recorded information is read out, the focusing lens is normally placed in a most widely spaced relation to the disk surface. If the focus servo loop were actuated in that state, no usable focus error signal could be obtained. Therefore, the focus servo loop is turned off (opened) at first, and a start signal is applied from an external source to energize the lens driving motor for moving the focusing lens to a position spaced approximately distance D.sub.0 from the disk surface. Then, the focus servo loop is turned on to enable focus servo control.
FIG. 1 of the accompanying drawings is a block diagram of a conventional arrangement of such a focus servo unit and a device for controlling the movement of a focusing lens. A focus error signal as defined above is applied through a servo loop switch 1 and an adder 2 to a positive input terminal of a drive amplifier 3 which issued an output signal through a buffer amplifier 4 as a drive signal to a motor M for moving a focusing lens (not shown). Prior to initiating the servo control mode, the adder 2 is supplied with a start signal from a controller (not shown) through a resistor R.sub.1 for a certain period of time.
The motor M and resistors R.sub.2 through R.sub.4 jointly constitute a bridge circuit having a pair of terminals a and b, between which the drive signal from the drive amplifier 3 is applied, and another pair of terminals c and d, the voltage between which is applied as an input signal to a differential amplifier 5. The differential amplifier 5 issues an output signal through a switch 6 and a resistor R.sub.5 to a negative input terminal of the drive amplifier 3. Resistors R.sub.6 and R.sub.7 constitute a negative feedback circuit for the drive amplifier 4.
In operation, when a focus ON signal is generated by the controller, the switch 6 is turned on and a start signal of a substantially constant level is applied through the adder 2 to the positive input terminal of the drive amplifier 3 thereby to energize the motor M. If the bridge circuit including the motor M is in a state of equilibrium at this time, one differential input terminal of the differential amplifier 5 is supplied with a voltage dependent on the counterelectromotive force generated across the coil of the motor M. While the switch 6 is closed, a servo loop composed of the bridge circuit including the motor M, the differential amplifier 5, and the drive amplifier 3 is formed and operates to maintain constant the speed of rotation of the motor M. Thus, a fixed-speed servo circuit arrangement is established for moving the focusing lens at a constant speed. As the focusing lens approaches the disk surface in the vicinity of the normal distance D.sub.0, the presence of the S-curve characteristic of the focus error signal is detected to turn on the servo loop switch 1 and turn off the switch 6. Therefore, the focus servo loop is actuated to thus control the motor M with the focus error signal in a normal focus servo control mode.
Where, however, the focus servo control is rendered unstable or the motor M is out of the focus servo control range due to some disturbance, the motor M is energized by the start signal applied from the controller for the certain period of time, thereby sometimes causing the focusing lens to excessively approach or hit the disk surface.
The present invention has been made to eliminate the foregoing difficulties of the conventional focusing device.
It is an object of the present invention to provide a device for controlling the movement of a focusing lens to thereby reliably prevent the focusing lens from impinging upon the disk.
In accordance with a device and method for controlling the movement of a focusing lens according to the present invention, the focusing lens is moved in a direction away from the disk if the device is not in a focus servo control mode after the focusing lens has been moved into a normal operative position. Thereafter, the focusing lens is moved again into the vicinity of the normal operative position.